(a) Field of the Invention
The present invention relates to a voltage multiplier having an intermediate tap and, more particularly, to a voltage multiplier for use in a LCD driver IC for driving a LCD panel.
(b) Description of the Related Art
A LCD driver IC for driving a LCD panel generally includes a power supply circuit outputting relatively high source voltages ranging between 5 and 20 volts. On the other hand, most current ICs including logic IC and memory IC use a relatively low voltage ranging between 3 and 5 volts. Thus, the power supply circuit in the LCD driver IC includes a voltage multiplier for multiplying a source voltage of 3 to 5 volts up to two to five times the supplied source voltage.
The power supply circuit in the LCD device, especially used in a personal digital assistant, generates a plurality of intermediate voltages in addition to the highest operatioanl voltage. FIG. 1 shows a typical power supply circuit including a conventional voltage multiplier.
The power supply circuit 30 includes a voltage multiplier block 31 for multiplying an input source voltage VDD by xe2x80x9cnxe2x80x9d to generate a multiplied voltage VLCD equal to nxc3x97VDD where xe2x80x9cnxe2x80x9d is an integer. The power supply circuit 30 also includes a voltage generator block 32 which receives the multiplied voltage VLCD to generate a plurality of voltage levels V1 to V5 including a highest voltage V1 and a lowest voltage V5.
The voltage generator block 32 includes a voltage divider 33 implemented by resistors R1 to R5, a voltage follower A1 which operate on a source voltage of VLCD and receives a reference voltage Vref to generate the voltage level V1, and a plurality of voltage followers A2 to A5 each of which operates on a source voltage of VLCD and receives one of output voltages of the voltage divider 33 to output a corresponding one of the voltage levels V2 to V5.
In the conventional power supply circuit 30, power dissipation can be suppressed by employing a higher resistance for the voltage divider 33, reducing penetrating current flowing through the output stages of the voltage followers A1 to A5, and stopping the bias current for the voltage followers A1 to A5 during a standby mode of the LCD device.
Due to the employment of lower power supply voltage, which is now reduced down to 1.5 to 2.0 volts, in the latest logic LSI, the multiplication factor of the voltage multiplier 31 has increased up to 6 to 9 times. Thus, further reduction of the power dissipation is requested in the power supply circuit 30 along with the current driveability of the voltage multiplier 31. The current driveability of the voltage multiplier 31 generally involves a larger chip area for the LCD driver, which is undesirable especially in a personal digital assistant.
Patent Publication JP-A-10-31200 describes a power supply circuit for a LCD driver such as shown in FIG. 2. The power supply circuit 40 includes a first voltage generator 41 for generating higher voltage levels V1 and V2, and a second voltage generator 42 for generating lower voltage levels V3 and V4. The first voltage generator 41 operates on a source voltage between a positive voltage level V0 of a power source 43 and a median voltage level Vx, whereas the second voltage generator 42 operates on a source voltage between the median voltage level Vx and the negative voltage level V5 of the power source 43.
The median voltage level Vx is generated from the positive voltage level V0, the negative voltage level V5 and bipolar transistors Tp and Tn. If the total output current from the operational amplifiers OP1 and OP2 constituting the voltage followers in the first voltage generator 41 is equal to the total output current from the operational amplifiers OP3 and OP4 in the second voltage generator 42, the median voltage Vx is maintained at the exact median potential between voltage levels V0 and V5. In this case, the power dissipation is reduced down to a half the power dissipation by the power supply circuit of FIG. 1.
In an ordinary LCD device, specified voltage levels are often used in the LCD panel at a time. For example, if the first voltage generator 41 operates for driving a LCD panel, electric charge generated by the driving is temporarily stored in a capacitor C1 connected between the median voltage line Vx and the negative source line. On the other hand, if the second voltage generator 42 operates for driving, the driving current reduces the charge stored in the capacitor C1. The capacitor C1 alleviates a voltage fluctuation on the median voltage line Vx. However, if the unbalance of the drive current continues, the median voltage line Vx may rise or fall from the median voltage by a threshold. The rise or fall of the median voltage line Vx above the threshold is cancelled by turn on of the bipolar transistor Tp or Tn.
The turn on of the transistor Tp or Tn results in flow of the current from the V0 line to the Vx line or from the Vx line to the V5 line, and causes a power dissipation. In addition, the function of the capacitor C1 is limited if the multiplication factor of the voltage multiplier increases.
It is therefore an object of the present invention to provide a voltage multiplier which is capable of reducing power dissipation in the case of providing an intermediate voltage level in addition to the multiplied voltage from the power supply circuit having the voltage multiplier.
The present invention provides a voltage multiplier including a plurality of capacitors, a switching assembly for iteratively switching between a first position for connecting the capacitors in parallel for charging the capacitors with a first voltage and a second position for connecting the capacitors in series to form a serial branch, the serial branch having a first end outputting a second voltage, at least one first switch having a terminal connected to an intermediate position of the serial branch, the first switch being turned on, when the switching assembly connects the capacitors in series, to output a third voltage which is higher than the first voltage and lower than the second voltage.
In accordance with the voltage multiplier of the present invention, since the power supplied on the third voltage is substantially equal to the corresponding power supplied on the first voltage, the power dissipation in a LCD panel, for example, driven by the voltage levels generated based on the third voltage is reduced compared to the LCD panel driven by the voltage levels generated based on the multiplied voltage generated by the conventional voltage multiplier.
The above and other objects, features and advantages of the present invention will be more apparent from the following description, referring to the accompanying drawings.